Not at all - and this is happening for multiple domains it didn't happen before fore. This is quite a regular DNS setup. The rule is only meant to match where there are no A or MX records associated with the domain at all

Note: envelope sender, which is the return path. As phoenix's second code block shows, the mail should pass this test. And of course if ALL mail is failing the test, then it's unlikely to be a problem with that one.

What I would do in this situation is run dig mozilla.org mx as zimbra on the Zimbra box. I'll bet you have a problem with your DNS configuration on your ZCS box or (more likely) on whatever DNS server it uses.

Note: envelope sender, which is the return path. As phoenix's second code block shows, the mail should pass this test. And of course if ALL mail is failing the test, then it's unlikely to be a problem with that one.

What I would do in this situation is run dig mozilla.org mx as zimbra on the Zimbra box. I'll bet you have a problem with your DNS configuration on your ZCS box or (more likely) on whatever DNS server it uses.

Thanks; that's helpful. It initially seemed like this was happening to all mail (within the initial period), but now I'm getting it on only some. I'm suspecting a transient/intermittent DNS failure; this helps me to investigate